1. Field of the Invention
The invention relates to a method of impregnation of foam cements, mortars and concretes with liquid monomers and resins, subsequent in situ polymerization of the liquid monomers or resins and thermal insulating structural elements, produced by the method, comprised of composite building components having an unimpregnated core and impregnated surface regions. The production method utilizes the flow under a hydraulic gradient or vacuum means of a liquid through an interconnected gaseous phase due to foam voids dispersed within the cement paste matrix. The gaseous phase is dispersed within the cement paste before the paste hardens, and is not water filled. Subsequent polymerization of the liquid in the dispersed void system results in a two-phase material with a large volume of polymer and having optimum thermal insulating and structural properties. The liquid flow is independent of the gel pores and capillary pores of the cement paste, which may therefore be water filled. The resistance of the interconnected gaseous phase to flow of a liquid is low relative to the resistance of the gel and capillary pores. Thus more viscous liquids may be used with this impregnation method than with normal concrete impregnation techniques. The use of viscous monomers reduces the dilution of capillary adsorption during partial impregnating processes.
The method has been used with foam neat cement pastes, foam mortars, and foam concretes and promoted polyester-styrene resin with peroxide catalyst. Uniformly sized spherical voids become interconnected at a foam void fraction of 0.524, which corresponds to a cubic packing of the spheres. The permeability of foam cements with foam void fractions above 0.6 is such that highly viscous monomers or resins can penetrate the foam cement. The dispersed voids are large enough to prevent capillary adsorption of the viscous liquid, and a driving pressure or vacuum is required for monomer or resin impregnation of the foam cement.
The relatively high permeability of lightweight foam concrete results in simple preparation techniques for liquid monomer and resin impregnation. Filling the foam voids with a polymer results in significant increases in strength and elastic modulus. Polymer loadings are high because of the large volume in the lightweight foam concrete.
Partly polymer impregnated foam concrete structural elements are a practical application because material with high polymer loading can be restricted to surfaces, which have high stresses and act as an interface with environment. The foam concrete core acts as an insulator and as a spacer for the polymer impregnated surface regions. Thus the partly impregnated foam concrete element or "sandwich" panel can satisfy the multiple functional requirements of load and of environment. A pilot study of the herein described concept of partly impregnated foam concrete structural elements was conducted at the U.S. Army Construction Engineering Research Laboratory as a part of the In House Independent Research Program. The study was in general limited to combinations of foam neat cements and a promoted polyester resin with MEK peroxide for a catalyst. This limitation was based on the premise that optimum combinations of foam cements and polymers may be determined once the feasibility of the concept has been verified and the important parameters have been identified. The results of the study were presented at a Conference on Radiation and Isotope Techniques in Civil Engineering, in Brussels on Oct. 28-30, 1970 and were published in a report No. 37 of the Commissioner of the European Communities, Bureau Eurisotop entitled Polymerized Lightweight Structural Elements by J. Lott and D. Birkhimer.
2. Description of the Prior Art
A joint research effort of the Brookhaven National Laboratory and the United States Bureau of Reclamation over the past 5 years has shown that impregnation of hardened portland cement concretes of normal weight by a monomer, which is subsequently polymerized in situ, results in a concrete with improved engineering properties, which include increased strength, modulus of elasticity, durability, and a decreased permeability. A portion of the voids in hardened cement is filled with the liquid monomer. The subsequent in situ polymerization of the monomer introduces a solid polymer phase into regions of the concrete that formerly had no strength.
The void system of hardened concrete is complex and usually consists of gel pores and capillary pores in the cement paste, entrained air, entrapped air, microcracks, and the pores in the aggregates. The gel pores are characteristic of the hydration product of portland cement and are estimated to be of the order of 10 to 20 Angstroms in width. The size, total volume, and distribution of the capillary pores are dependent on the original water-cement ratio and the degree of hydration that has occurred. The capillary pores which vary in size, are estimated to be of the order of 5.times.10.sup..sup.-5 inches and form a random void system, which is often interconnected. The other voids present in concretes are usually larger in size than the capillary pores.
Monomer impregnation is probably limited to a small portion of the gel pores, to the capillary pores, and to the larger voids. Impregnation is retarded by the presence of evaporable water in the voids. Maximum monomer loading of normal weight concretes, which BNL and USBR have found essential for optimum engineering properties, requires the removal of evaporable water by thermal or vacuum drying and evacuation of the concrete to remove air from the voids before monomer soaking. Pressure is often used to reduce impregnation times. Researchers at the American Cement Corporation, Technical Center, Riverside, Calif. have increased the volume of capillary pores present in cement paste by using high water-cement ratios to accomplish impregnation by capillary adsorption. This technique requires the removal of evaporable water, and specimens were dried under vacuum at 85.degree. C for 24 hours before impregnation.
The techniques for obtaining maximum polymer loadings are complex. Methods for vacuum drying, thermal drying, and evacuation of concrete limit the practical applications of the resulting two-phase material. Only monomers with viscosities that are approximately equal to the viscosity of water have been used successfully to impregnate concretes with a normal weight cement paste matrix. Partial impregnation is difficult with these techniques since capillary adsorption removes the liquid monomer from the impregnated region and results in less than maximum polymer loadings in the regions of desired impregnation.
U.S. Pat. No. 3,567,496 concerns a method for impregnating preformed unfoamed concrete with a monomer and a peroxide polymerization catalyst and heating the concrete body until the monomer has polymerized in situ. Decreased permeability and increased compressive strength are claimed for the product. The concrete must be dry before impregnation and the monomer and catalyst may be impregnated in the same operation. Impregnation can be accomplished by immersing the concrete body and pressure can be used to increase the rate of impregnation. The degree of penetration can be varied by preevacuation, pressurization soaking, gas phase saturation, etc.
The improvement of using foam masonry compositions as in the instant invention results in a higher polymer loading because of large volume of interconnected foam voids, a very low pressure, polymerization of a higher viscosity monomer at room temperature and partial impregnation since capillary adsorption, which removes less viscous liquid monomers from the impregnated region, is prevented. Tests on samples of foam masonry polymer impregnated as in the instant invention showed compressive strength increase of 15 times and tensile strength increase of 32 times that of a non-foam polymer impregnated cement.
U.S. Pat. No. 2,751,775 teaches coating and impregnating a surface of a masonry block with a coating comprising polyester resin, styrene monomer and a peroxide catalyst by placing a masonry block in a mold containing the above coating composition and allowing the weight of the block to cause the composition to penetrate the pores of the masonry block. It is obvious that monomers of high viscosity would not impregnate a masonry block by this method and that impregnation of a considerable depth would not be possible nor would a high polymer loading of the masonry material be possible.
Foamed concrete and foamed portland cement are known for example as in the Third Topical Report on Concrete-Polymer Materials dated January 1971 by the Bureau of Reclamation, Denver, Colo., Reference REC-ERC-71-6 and BNL 50275 (T-602). This reference employs foamed-glass aggregate concrete impregnated with methyl methacrylate and mearlcreate, a foamed portland cement containing no aggregate phase, which latter preformed foam is used in the instant invention. However, using the less viscous methyl methacrylate, 20-30 times water, does not permit control of impregnation of the concrete by the polymerized liquid that the method of the instant invention does. The improvement of filling the foam voids with a considerably more viscous impregnating liquid to a considerable depth below the surface of a cast form yields a greater flexural stress capability and high thermal insulating properties. The improvement described herein employs mearlcrete, not as a portland cement with aggregate but as a foam material to create voids in cement which voids are filled with a polymerizable liquid of a viscosity as high as 700 times that of water. The product obtained by the method herein disclosed yields an optimum condition in a material i.e. high insulating value and high structural strength.